Device having a shaped, magnetic toroidal member and a magnetoresistive sensor for detecting low magnitude electrical currents

ABSTRACT

A device for detecting low magnitude electrical currents includes a generally toroidally-shaped member made of magnetic material that provides an air gap, and a magneto-resistive device in the form of a bridge located in the air gap for sensing a current flowing through a conductor that passes through the member. In order to reduce damage due to overcurrents, the member has a portion of reduced cross-sectional area to cause saturation of the member. The device can be used in an earth leakage current detector.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the detection of electrical currentsand more particularly to a device for detecting low magnitude electricalcurrents.

2. The Prior Art

There are a number of situations where it is necessary to detectelectrical currents so that they can be monitored, and there have been anumber of different proposals put forward for achieving this. In thefield of residual current devices, it is customary to detect animbalance in current flow through live and neutral conductors by passingthe conductors through a toroid of magnetic material and having asensing winding around part of the toroid. This arrangement is such thatif there is an earth leakage fault, there is an imbalance between thecurrent flowing in the live conductor with respect of the currentflowing in the neutral conductor, and this imbalance results in acurrent being induced into the sensing winding. This winding is thenconnected to suitable circuitry for indicating the presence of the faultcondition.

Residual current devices have become more and more popular and there isconsequently a continuing need to be able to manufacture such devices inas efficient and cost effective a manner as possible. Attention hastherefore been directed to the need for a sensor coil to be wound on atoroid as this is a relatively expensive operation.

European Patent Application EP-A-0242560 describes an arrangement fornon-contact measurement of the electric current in a conductor, andrelates to a device having a magnetic core surrounding the conductor. Ina gap in the magnetic core is a gap in which is positioned a magneticfield sensor. The aim of this prior invention is to increase themeasuring sensitivity of the device, and this is achieved preferably byutilising a magnetic core whose cross-sectional area becomes greaterwith increasing distance from the gap.

It is known from the article "Stromsensor als Mikrosystem" (Elektronik,vol. 42 No. 5 October 1993. Munchen, DE; pages 42-44, XP 000396422; Ruhlet al) that different types of devices may be placed in the air gap of atoroidailly-shaped member in order to sense currents.

SUMMARY OF THE INVENTION

The present invention proposes to use a magneto resistive device toreplace the sensor coil in an RCD device, for example. However, the useof a magneto resistive device is not without its problems, andconsequently the present invention further provides a generallytoroidially-shaped member which is arranged to saturate at apredetermined level of magnetic flux.

According to the present invention there is provided a device fordetecting a low magnitude net electrical current comprising a generallytoroidally-shaped member of magnetic material provided with an air-gapand a sensor disposed in the air-gap for sensing a magnetic fluxindicative of the net current flowing in conductor means passing throughthe toroidally-shaped member, characterised in that

the sensor is a magneto-resistive device; and

at least a portion of the toroidally-shaped member is shaped whereby tocause saturation of the member at a predetermined level of magneticflux.

In order that the present invention be more readily understood, anembodiment thereof will now be described by way of example withreference to the accompanying drawings in which

FIG. 1 is a diagrammatic perspective view showing the generalarrangement according to the present invention;

FIG. 2 shows one form of a toroidially-shaped member for use with thepresent invention; and

FIG. 3 shows an alternative form of the toroidially-shaped member foruse with the present invention.

Magneto resistive detectors are known and in particular magnetoresistive bridge devices have been produced which will change theirresistance as a function of applied magnetic field. A characteristic ofexisting devices is that they are only linear over a small proportion oftheir range and further it is necessary to bias each resistor of thebridge using a permanently magnetic coating. In the past, this has meantthat magneto resistor devices have been restricted to uses where theamount of flux to be detected has been small enough to ensure that thepermanent magnetic coating would not be destroyed as might result iflarge magnetic fields were to be impressed upon the device. This isparticularly a problem when one is considering residual current deviceswhere one is attempting to monitor fault currents of the order of 30milliamps but it is possible that currents of the order of amps mightwell pass through the conductors which would in turn impress very largemagnetic fields on the magneto resistive device.

If one now turns to FIG. 1, the general arrangement of a sensor for aresidual current device is shown and it will be seen that it comprises amagnetic field enhancement device in the form of a toroidially-shapedmember 10 through which pass conductors 12 which are respectivelyconnected to the live and neutral supplies to the residual currentdevice. The toroidially-shaped member 10 is provided with an air gapindicated by the reference numeral 14 and a magneto resistive device inthe form of a magneto resistor bridge 16 is disposed at the air gap soas to be affected by the flux passing between the faces 14a, 14b of thetoroidially-shaped member. This is a necessary feature since the MRelement requires an in plane applied field for optimised field sensing.The MRs employed in the sensor, are fabricated in a serpentine shapefeature so as to increase the total resistance per unit area. Thesensors were fabricated using RF sputtering and standardphotolithography processes with track widths of the sensor being 110 μm,with an inter-element gap or 2 μm. The track width is designed to reducethe effect of demagnetising fields at the edge of the thin film tracksand thus increase the sensitivity of the sensor [2]. The increase insensitivity leads to a decrease in the saturation fields of sensor. Thisfeature may be ultimately designed for different ranges of sensitivityand saturation by employing varying track widths.

With current flowing in the mains conductor, an in plane magnetic fieldis applied to the sensor. The intensity of this field is directlyproportional to the magnitude and direction of the current in the wireand the magnetic circuit, coupling with the field generated due to theconductor. The four elements of the bridge are complementary in theircharacteristics, providing for thermal drift reduction. The change inthe overall resistance of the MR bridge is a function of the appliedfield. Signals from the bridge may be AC coupled to an instrumentationtype amplifier, so as to reduce zero error. The sensors were suppliedwith a low noise battery operated current source.

The conductors 12 are preferably twisted together at least in the areaof the toroidially-shaped member 10 so as to ensure equalization ofcurrents and the arrangement is such that in the absence of a faultcondition, there is no magnetic flux in the air gap 14. However, in thepresence of an earth leakage fault there is an imbalance in current flowthrough the conductors 12 which in turn causes a magnetic flux to becreated in the toroidially-shaped member. The magneto resistor bridge 16detects the presence of the magnetic field in the air gap and produces asignal which can then be processed in the usual way. Thetoroidially-shaped member 10 is made of a high permeability material soas to provide the necessary sensitivity bearing in mind that the faultcurrents used to induce a magnetic field are of the order of 30milliamps or less. Additionally, the toroidally-shaped member 10 isshaped to cause the member to saturate and this is most readily achievedby producing a reduced cross-sectional area portion as indicated by theletter A in FIG. 2 or FIG. 3 as compared with the cross sectional areaof the faces 14a and 14b. It will be noted that the reducedcross-sectional area is remote from the air gap. By careful design ofthe reduced cross-sectional area, it is possible to limit the amount offlux in the air gap to a level such that damage to the magneto resistorbridge device 16 will be avoided under all conditions of current.

It is intended that the overall device will be housed in a metal box toprotect the toroid and magneto resistor bridge from strayelectromagnetic fields.

I claim:
 1. A device for detecting a low magnitude electrical currentcomprising a generally toroidally-shaped member of magnetic materialprovided with an air-gap and a sensor disposed in the air-gap forsensing a magnetic flux indicative of the current flowing in conductormeans passing through the toroidally-shaped member, whereinthe sensor isa magneto-resistive device; and at least a portion of thetoroidally-shaped member is shaped to cause saturation of the member ata predetermined level of magnetic flux and prevent damage to saidmagneto-resistive sensor device.
 2. A device according to claim 1wherein a portion of the member shaped to cause saturation of the memberis a portion of reduced cross-sectional area.
 3. A device according toclaim 2, wherein the portion of reduced cross-sectional area extendsonly over a portion of the toroidally-shaped member remote from theair-gap.
 4. A device according to claim 1, wherein the magneto-resistivedevice is in the form of a bridge.
 5. A residual current deviceincluding a device for detecting low magnitude net electrical currentsaccording to claim 1.